The present invention relates to an improved electrostatic copying machine comprising a reciprocating scanning mechanism.
An electrostatic copying machine of the present type comprises a document supporting platen and a scan unit having a light source and scanning element such as a mirror. Either the platen or the scan unit is reciprocated relative to the other in a scan stroke and a return stroke.
In order to ensure that the trailing edge of the original document in the scan direction is illuminated with the same intensity as the remainder of the document, the light source is energized for a certain period of time after the scan stroke is finished and the return stroke has begun. This results in a portion of a photoconductive drum being imaged with not only the desired image of the document but also a partial reversed image due to energization of the light source during the first part of the return stroke. This partial reversed image must be removed by a drum cleaning unit and may overload the cleaning unit if the partial reversed image contains a large proportion of dark areas. Where the length of the image in the scan direction is smaller than the length of a copy sheet and the return stroke is begun as soon as the trailing edge of the document has been scanned, the partial reversed image will be transferred onto the copy sheet. Both of these problems which have existed heretofore in the prior art are undesirable and arise from the same cause.